Optical fibers of this type are generally known; they are mainly used in the field of telecommunication. Refer, for example, to European patent application No. 0 127 227, U.S. Pat. No. 5,242,476 and U.S. Pat. No. 5,838,866. Because of their characteristically low attenuation and dispersion, such optical fibers are in particular suitable for forming long-distance data links, which frequently bridge several thousand kilometers. When bridging such large distances, it is of major importance that the cumulative signal losses in the optical fiber be minimized if transmission of optical signals is to take place with a small number of intermediate amplification stations. With the transmission wavelength of 1550 nm that is generally used, it is a general requirement of the telecommunication industry that a total attenuation value in such optical fibers of 0.25 dB/km, preferable 0.2 dB/km, is not exceeded.
Although the fibers that are currently being manufactured are capable of meeting such requirements with regard to the allowable attenuation, it is nevertheless frequently observed that with the passage of time the same optical fibers exhibit significant increases as regards the attenuation that occurs therein. Extensive research has shown that this phenomenon can be attributed to the gradual ingress of hydrogen gas from the surrounding atmosphere into the fiber, resulting in the formation of groups such as SiH and SiOH within the fiber. These compounds exhibit a strong infrared absorption, with attenuation peaks at wavelengths of about 1530 and 1385 nm.
A solution for overcoming the problem of such hydrogen induced attenuation is known from European patent application No. 0 477 435. According to the method that is known therefrom, an optical fiber is extensively exposed to a hydrogen-containing gas during its manufacture, in order to ensure that all structural defects sites in the fiber are already provided with a hydrogen atom before actual implementation of the fiber takes place. One drawback of this known method, however, is the fact that it only tackles the symptoms of hydrogen-induced attenuation rather than the causes thereof. In addition, this known measure complicates the production process to a significant degree and introduces an additional risk of contamination of the fiber product by the hydrogen-containing gas that is used.
In addition to that, Dutch patent application NL 1015405 in the name of the present inventors, which has not been laid open to public inspection yet, discloses the possibility of preventing a significant increase of the hydrogen-induced attenuation at a wavelength of 1550 nm by building up the internal cladding of the optical fiber from SiO2 doped with fluor in an amount of 0.1-8.5 wt. %, so the core is subjected to an axial compressive stress over the entire cross-section thereof, which axial compression suppresses the occurrence of defects.